Recently, the food packaging art has undergone remarkable technical innovation. For example, it is well known that hydrophobic thermoplastic resins such as saturated polyesters and the like are being put to practical use in food packaging to improve hygienic qualities and provide beautiful appearance as well as to save weight for cutting the cost of transport. However, such saturated polyester resins have low gas barrier properties against oxygen, carbon dioxide and the like and, therefore, are not suitable for long-term storage. Accordingly, glass bottles and metallic cans are predominantly used, resulting environmental problems when used cans are discarded, increase in cost for recovering used bottles and the like. In view of this, it is desired to develop other suitable materials for packaging.
EVOH containing 20-55 mole % of ethylene component has excellent properties suitable for food containers and packaging materials such as melt molding characteristics, high transparency and excellent gas barrier properties. However, other properties thereof, for example, stiffness and resistance to moisture are defective and insufficient.
Hydrophobic thermoplastic resin, particularly, a saturated polyester resin mainly composed of polyethylene terephthalate is excellent in molding characteristics, in the balance between impact resistance and stiffness, in hygienic qualities and the like. However, as described above, its gas barrier properties against oxygen, carbon dioxide and the like thereof are insufficient, and its use for food containers and packaging materials is limited.
Therefore, in order to impart both good gas barrier properties of EVOH and excellent mechanical properties of a hydrophobic thermoplastic resin such as a saturated polyester resin to food containers and packaging materials, lamination of both resins may be considered. However, both resins have little affinity for each other and it is not possible to obtain a laminated material of both resins by simple thermal adhesion.
Japanese Patent Kokai No. 55-71556 discloses good adhesion between a polymer composed of ethylene component-acrylate or vinyl acetate component-ethylenic unsaturated carboxylic acid or its acid anhydride component, and a saturated polyester resin. This publication shows that a modified ethylene-carboxyl group containing monomer copolymer, wherein an ethylenic unsaturated carboxylic acid or its acid anhydride is chemically bonded to a copolymer composed of ethylene and a carboxyl group containing monomer, such as ethylene-vinyl acetate copolymer or ethylene-acrylate copolymer, has good adhesion to a saturated polyester resin. The utility thereof is also disclosed in the publication.
Japanese Patent Kokai No. 54-101883 discloses that a thermoplastic resin and EVOH can be laminated by using as an intermediate layer a modified ethylene-carboxyl group containing monomer copolymer, wherein an ethylenic unsaturated carboxylic acid or its acid anhydride is chemically bonded to a copolymer composed of ethylene and a carboxyl group containing monomer. In addition, in Japanese Patent Kokai No. 54-110282, a laminated structure containing EVOH is obtained by using as an adhesive layer a metal-combining modified copolymer which is produced by chemically bonding an ethylenic unsaturated carboxylic acid or its acid anhydride to a copolymer composed of ethylene and a carboxyl group containing monomer, and reacting with a metallic oxide or carbonate.
However, when a copolymer composed of ethylene component-acrylate component or lower fatty acid vinyl ester component-ethylenic unsaturated carboxylic acid or its anhydride component, or a resin obtained by reaction of such a copolymer with a metallic oxide or a carbonate is used as an adhesive layer in a combination of EVOH and a hydrophobic thermoplastic resin such as a saturated polyester resin mainly composed of polyethylene terephthalate, the resulting material has less commercial value, even though adhesion is effected to some extent. For example, the laminated material obtained has inferior appearance due to formation of streaks, hard spots, wave patterns, etc., the main cause of which is considered to be nonuniformity in thickness of the layer of resin having gas barrier properties and formation of a gelled material.
Further, Japanese Patent Kokoku No. 39-6810 discloses a process for preparing an ionic copolymer wherein one or more water soluble ionic metallic compounds are reacted with a copolymer obtained from one or more a .alpha.-olefins and one or more .alpha..beta.-ethylene type unsaturated carboxylic acids which have one or two carboxyl groups. In this publication, it is taught that a preferred base copolymer is that obtained by directly copolymerizing ethylene with a monocarboxylic acid monomer in the presence of or in the absence of an additional copolymerizable monoethylene type unsaturated monomer. This publication teaches that, when 10% or more of the acid group of this base copolymer is neutralized with a metallic ion such as Na.sup.+ and the like, both desired melt-flow characteristics as well as desired solid state physical properties such as rubber elasticity, flexibility, thermal stability, tenacity, wear resistance and the like can be imparted to the resulting material. This is advantageous to the material obtained in this publication. On the other hand, although it is also taught that the base copolymer can be obtained by graft-polymerizing .alpha.-olefin and an acid monomer with a polyolefin base, it is explained that this process is suitable for using a polyolefin obtained from an olefin having a molecular weight larger than that of ethylene, for example, propylene, butene-1, etc., and there is no disclosure that ethylene-acrylate or vinyl acetate copolymer can be used as a polyolefin. Further, the invention disclosed in this publication relates to improvement of physical properties such as those described above in a resin having a melt index (MI) of 0.1-100 g/10 min., and it is disclosed therein that the resin can be used as an adhesive or for a laminated material, when the MI thereof is 100 g/10 min. or more.
However, an ionic copolymer obtained by reacting a copolymer composed of one or more .alpha.-olefins and one or more .alpha..beta.-ethylene type unsaturated carboxylic acids which have one or two carboxyl groups, with one or more water soluble ionic metallic compounds does not exhibit adhesion at all regardless of the presence of acrylate or vinyl acetate component. Further, when the MI is 100 g/10 min. or more, there are drawbacks such as difficulty in molding due to too large an MI. Therefore, the adhesive resin is not practically useful.
In Japanese Patent Kokai No. 54-87783, it is disclosed that, when a polyolefin obtained by modification of a part or all of the polyolefin with at least one monomer selected from unsaturated carboxylic acids and derivatives thereof is laminated with EVOH, a polyamide resin or a polyester resin, interlaminar adhesion can be improved by incorporating at least one metallic compound selected from the group consisting of metallic salts of higher fatty acids, metallic oxides, metallic hydroxides, metallic carbonates, metallic sulfates and metallic silicates into at least one layer. The polyolefin disclosed in this patent publication is a homopolymer of .alpha.-olefin such as ethylene, propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene and the like, or a copolymer of at least one monomer and other .alpha.-olefins. It is disclosed in the publication that the advantage of the invention extraordinary inter alia in medium or high density polyethylene and polypropylene. Ethylene-vinyl acetate copolymer is also disclosed.
However, in Japanese Patent Kokai No. 54-87783, there is no disclosure relating to an ethylene-acrylate copolymer and an ethylene-methacrylate copolymer.